Generally, in magnetic recording mediums obtained by dispersing ferromagnetic powder, a binder, a curing agent and other additives in an organic solvent, and coating the resulting coating composition on a nonmagnetic support and drying it, the magnetic layer thereof may be scraped as it slides by a magnetic head, etc. Accordingly, it is necessary that the magnetic layer is scarcely abraded and has excellent durability. To meet this need, it has been conventional to use a polyurethane binder which is excellent in abrasion resistance and toughness. Alternatively, the strength of the layer is increased by adding a curing agent to the binder and carrying out a crosslinking reaction, or an adsorptive polar group, such as a sulfonic acid group, a phosphoric acid group, a carboxyl group or a derivative thereof, is introduced into the binder to form adhesion between the ferromagnetic powder and the binder.
The sliding of the magnetic head on the magnetic recording medium is greatly affected by the binder present on the surface of the magnetic layer and in the vicinity of the surface thereof. Even when ferromagnetic powder is subjected to an adsorption treatment or when a crosslinking reaction of the binder is carried out, the low-molecular components in the uncrosslinked or unadsorbed binder migrate to areas in the vicinity of the surface of the magnetic layer. As a result, problems such as clogging of the head and staining of the guide pole are caused.
To cope with the problem, the present inventor has previously proposed that, for example, in a multi-layer structured magnetic recording medium comprising a non-magnetic support having thereon a lower magnetic layer and an upper magnetic layer, the binder used in the upper magnetic layer has a molecular weight which is higher than that of the binder used in the lower magnetic layer. As a result, the strengths of the layer can be increased and at the same time, running durability can be improved [see, JP-A-1-263925 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")]. Further, JP-A-1-205723 discloses that in a magnetic recording medium having the structure described above, the amount of the polyisocyanate contained in the upper magnetic layer is larger than that of the polyisocyanate contained in the lower magnetic layer, to thereby improve running durability.
In the former method, however, since a binder having a high molecular weight is used, there is the problem that the magnetic layer is excessively hardened and as a result, there is a difficulty in satisfactorily calendering the magnetic layer. Additionally, since the molecular weight of the binder is originally too high, the ferromagnetic powder is poorly dispersed, and, as a result, good surface properties can not be obtained and it is difficult to obtain a high RF output.
In the latter method, since the amount of the polyisocyanate to be contained in the upper magnetic layer and the molecular weight of the binder to be reacted with the polyisocyanate must be properly controlled, there is the problem that it is difficult to prepare an upper magnetic layer having optimum hardness.
The problem of running durability in particular has become a serious problem with the development of higher-density recording and higher performance of the magnetic recording medium in recent years.
Accordingly, there is a need for an effective means for obtaining stable, good running durability in the magnetic recording mediums.
The present inventors have made studies with the view to solve the problems mentioned above. In the studies, materials by which the head was clogged and the reaction system of the resin component of the binder with the curing agent component in the magnetic layer were quantitatively analyzed by means of ESCA, FT-IR, EPMA, GPC, etc. As a result, an effective means of solving the above problems has been found.